1. Field of the Invention
This invention relates to light emitting compositions and light-emitting devices that include the light-emitting compositions. Specifically, this invention relates to light emitting compositions and light-emitting devices that include a light-emitting lumophore-functionalized nanoparticle.
2. Description of the Related Art
Organic electroluminescent devices capable of emitting white light are desirable because of their potential utility as backplane lights for displays, overhead lighting and other lightweight, low profile, low power lighting applications. White light-emitting Organic Light-Emitting Diode (OLED) devices with high color purity and brightness exceeding 2000 cd/m2 have been demonstrated at least since 1994. (1, 2) However, there is considerable difficulty in preparing white emitting OLEDs because it is generally quite difficult to prepare a device with a single layer that can emit white light. Several ineffective strategies have been employed to generate white light by electroluminescence including: preparation of devices with multiple emitting layers, e.g. red, green and blue (2); use of a single emitting layer doped with multiple small molecule emitters of different colors (1, 3, 4); blends of different color emitting polymers (5, 6); excimer (7) or “electromer” (8) emission from a semiconducting polymer; excimer emission from an interface (9); and broad emission from metal chelates (10).
There are significant drawbacks to all of these approaches. Preparation of devices with multiple emitting layers is typically more difficult and time consuming than preparation of devices with fewer layers. Device failure is more likely to occur due to interfacial defects, and matching the conduction band energies of multiple layers is complicated at best. Small molecules tend to have limited solubility in polymers. Blends of small molecule emitters and polymer dispersions of emitters tend to aggregate or phase separate, which often results in decreased device performance and poor color stability. Classical polymer-based systems are typically difficult to purify and exhibit poor batch-to-batch reproducibility. It is also very difficult to control the structure of classical polymer-based systems except in a very general sense. Finally, broad spectral emission from small single molecules typically heavily consists of green wavelength components and has a much lower efficiency for the red and blue components. The human eye is most sensitive to green light; hence in an actual device, it is desirable to have the red and blue wavelength components brighter than the green components. Molecular orbital and quantum mechanical theories forbid this type of emission from a single small molecule material.
Recently, phosphorescent dyes have been used as a source of emission in OLEDs because of their potential for achieving high degrees of luminescence efficiency. In theory, phosphorescence can achieve 100% quantum efficiency by emitting from both the singlet and triplet state as compared to fluorescence which only emits from the singlet state and is thus limited to a theoretical efficiency of 25% (11).
Two of the most common techniques for achieving white light using phosphorescent organic light emitting diodes (PHOLEDs) are the co-doping of red, green, and blue phosphors into a single emission layer (12) and the building up of a multilayer device with each layer containing a different color phosphor (13). While a single emission layer can be easy and cost effective, the presence of multiple dopants in the same layer can result in problems with energy transfer. For example, energy transfer from the high energy dopants to the low energy dopants makes color balance difficult. Although segregation of the various emitters into separate layers helps to overcome the energy transfer problem, multilayered devices are much harder to fabricate and minor changes in layer thickness will result in significant changes in color balance.
The following articles are referred to above and incorporated by reference herein in their entireties:    1. Kido, J., Hongawa, K., Okuyama, K. & Nagai, K. White light-emitting organic electroluminescent devices using the poly(N-vinylcarbazole) emitter layer doped with three fluorescent chromophores. Applied Physics Letters 64, 815 (1994).    2. Kido, J., Kimura, M. & Nagai, K. Multilayer White light-Emitting Organic Electroluminescent Device. Science 267, 1332-1334 (1995).    3. Kido, J., Ikeda, W., Kimura, M. & Nagai, K. Jpn. J. Appl. Phys. (part 2) 35, L394 (1996).    4. Tasch, S. et al. Applied Physics Letters 71, 2883 (1997).    5. Yang, Y. & Pei, Q. Journal of Applied Physics 81, 3294 (1997).    6. Granstrom, M. & Inganas, O. Applied Physics Letters 68, 147 (1996).    7. Gao, Z. Q., Lee, C. S., Bello, I. & Lee, S. T. White light electroluminescence from a hole-transporting layer of mixed organic materials. Synthetic Metals 111-112, 39-42 (2000).    8. Lee, Y.-Z. et al. White light electroluminescence from soluble oxadiazole-containing phenylene vinylene ether-linkage copolymer. Applied Physics Letters 79, 308-310 (2001).    9. Chao, C.-I. & Chen, S.-A. White light emission from exciplex in a bilayer device with two blue light-emitting polymers. Applied Physics Letters 73, 426-428 (1998).    10. Hamada, Y. et al. White light-emitting material for organic electroluminescent devices. Jpn. J. Appl. Phys. (part 2) 35, L1339-L1341 (1996).    11. Baldo, M. A.; O'Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.; Thompson, M. E.; Forrest, S. R. Nature 395, 151 (1998).    12. D'Andrade, B. W.; Holmes, R. J.; Forrest, R. S. Adv. Mater. 16, 624 (2004).    13. Cheng, G.; Zhang, Y.; Zhou, Y.; Lin, Y.; Ruan, C.; Liu, S.; Fei, T.; Ma, Y.; Cheng, Y. Appl. Phys. Lett. 89, 043504 (2006).